1. Field of the Invention
The present invention relates to an acceleration sensor which may be advantageously used in a webbing retractor that is installed in a vehicle and is adapted to detect any emergency experienced by the vehicle from the acceleration acting in the horizontal direction and to suspend the unwinding of the webbing upon detection.
2. Description of the Related Art
A webbing retractor of the above-described type has an acceleration sensor provided for sensing acceleration of the vehicle in the horizontal direction. An arrangement of such an acceleration sensor is known in which the sensor has a mass body which is movable on an inclined surface, and, when the mass body moves in the horizontal direction to climb the inclined surface, a lever-shaped pawl is actuated by the action of the mass body to suspend the rotation of a lock ring, thereby instantly suspending the unwinding of the webbing from the webbing retractor.
With this arrangement of the acceleration sensor, however, the pawl is also actuated when the mass body moves in the vertical direction. With the above-described acceleration sensor, therefore, when acceleration acts in the vertical direction, the unwinding of the webbing may be suspended even if no emergency is experienced by the vehicle.
Such a phenomenon often takes place when the vehicle is running on a rough surface or if the vehicle is such a vehicle as a motor-truck having a suspension with a large sprig constant. More specifically, the phenomenon occurs when a factor such as bouncing of the vehicle causes the mass body to jump upward from the inclined surface or undergo rolling movement on the inclined surface. In such a case, the vertical acceleration is very much greater than horizontal acceleration of an ordinary magnitude.